1. Field of the Invention
The present invention relates to a film forming method including a purging process for purging a reaction vessel with a purging gas after completing a film forming process for forming, for example, a silicon nitride film on a surface of a substrate, a film forming system for carrying out the film forming method, and a recording medium storing programs to be executed by a computer to carry out the film forming method.
2. Description of the Related Art
A semiconductor device fabricating process includes a film forming process for forming a silicon nitride film, namely, a film of Si3N4 (hereinafter referred to as “SiN film”) on a substrate, such as a semiconductor wafer (hereinafter, referred to as “wafer”) W. The SiN film is formed on each of wafers W by a chemical vapor deposition method (CVD method) using a batch type thermal processing system. The SiN film is formed a surface of each of the wafers W by carrying a wafer holder holding the wafers W in layers into a vertical reaction vessel of quartz, keeping the interior of the reaction vessel at a predetermined pressure and supplying process gases for depositing the SiN film into the reaction vessel.
A main product and by-products of a film forming reaction deposit in films on the inside surface of the reaction vessel and the wafer holder after the thermal processing system has repeated the film forming process for forming the SiN film. The deposited films grow gradually as the number of cycles of the film forming process increases. After the thickness of the deposited films has increased to a predetermined thickness, the deposited films produce gases, the deposited films are cracked or the deposited films come off the surfaces in particles when the reaction vessel is heated for the next cycle of the film forming process.
To avoid such troubles, a cleaning gas is supplied into the reaction vessel to remove the deposited film deposited on the reaction vessel completely by the etching effect of the cleaning gas after the film forming process has been repeated by a predetermined number of cycles. If the reaction vessel is cleaned by etching after every cycle of the film forming process, the reaction vessel of quartz is deteriorated and it takes a considerably long time to remove the cleaning gas completely from the reaction vessel. Therefore, a purging process is executed after the completion of every cycle of the film forming process.
The purging process is carried out after the wafer holder holding the processed wafers W has been unloaded from the reaction vessel and before the wafer holder holding unprocessed wafers W is loaded into the reaction vessel. In the purging process, an empty wafer holder not holding any wafers W is carried into the reaction vessel, the interior of the reaction vessel is kept at a predetermined pressure and at a predetermined temperature, a purging gas, such as nitrogen gas (N2) is supplied into the reaction vessel, the reaction vessel is cooled rapidly, is evacuated and heated to remove surface layers of the films adhering to the inner surface of the reaction vessel positively by the purging process to suppress the generation of gasses and the production of particles. The purging process thus removes the surface layers of the deposited films are about to come off the reaction vessel. Removal of the surface layers of the deposited film is effective in suppressing the production of gasses and particles in the film forming process to be executed subsequently to the purging process.
The purging process is controlled on the basis of a fixed purging recipe regardless of the type of the film forming process carried out in the reaction vessel and the thickness of the film formed of films successively deposited on the inside surface of the reaction vessel. Time necessary to complete the purging recipe is on the order of 50 min. The purging recipe is designed for the strongest purging process for removing a very thick deposited film or a film of the type that is difficult to remove. Therefore, the production of gases and particles from most deposited films can be suppressed without continuing the purging process for such a long time specified by the purging recipe and hence some part of the purging process is unnecessary. The time spent for the unnecessary part of the purging process is a downtime of the film forming system. Increase in the downtime decreases the throughput of the film forming system. The purging process includes the steps of cooling the interior of the reaction vessel at a predetermined temperature, heating the interior of the reaction vessel and evacuating the reaction vessel. Thus the unnecessary purging process increases the processing cost.
Such a problem may be solved by modifying the purging recipe for each cycle of the film forming process taking into consideration the estimated amount of particles that may be produced in the reaction vessel. However, much time is needed for the analysis of the causes of particle production and, consequently, time necessary for the purging process increases.
A technique for reducing the amount of particles adhering to the wafers W is proposed in Patent document 1. This previously proposed technique cracks the deposited films adhering to the inside surface of a reaction furnace by varying the temperature in the reaction furnace and makes a purging gas carry particles produced when the deposited films are cracked outside the reaction furnace. However, this previously proposed technique uses the same purging recipe regardless of the type of the film forming process and hence cannot solve the foregoing problem.
Patent document 1: JP 200-306904 A